Initial steps of particle formation and growth

Aerosol particles are ubiquitous in the Earth's atmosphere and influence our quality of life in many different ways. In urban environments, aerosol particles can affect human health through their inhalation (e.g. Wichmann and Peters, 2000). In the global troposphere aerosol particles are thought to contribute to climate change patterns (Stott et al., 2000; Ramanathan et al., 2001). Understanding these effects requires detailed information on how aerosol particles enter the atmosphere and how they are transformed before being removed by dry or wet deposition. Key processes in this respect are the formation of new atmospheric particles and their subsequent growth to larger sizes. Quantitative measurements of aerosol formation and growth rates have required the recent developments in instrumentation for measuring size distributions down to sizes as small as 3 nm in diameter (McMurry, 2000). However, the phase change between vapour and liquid will occur at smaller sizes, typically around 1 nm in diameter. Thus, e.g. DMPS/SMPS systems, which have cut off sizes of 3 nm, are not suitable for direct detection of nucleation and initial steps of the particle growth. Ion spectrometers are one class of experiments that can reach this important size range in which the initial formation is occurring (H”rrak et al., 2000). The ion spectrometers reveal the mobility spectrum, and thus also indirectly the size distribution, of atmospheric ions. Here were report the first field experiments where ion spectrometers (two of them) have been used together with DMPS to study formation and growth of aerosol particles focusing particularly in the initial steps of the growth. In principle the initial steps of the growth can occur via several ways: condensation of nucleating vapours, activation of soluble vapours (multicomponent nano-Kohler theory), heterogeneous nucleation or ion mediated particle formation (Yu and Turco, 2000). In this paper, we demonstrate how the analysis of ion distribution data, together with DMPS data, can result in useful hints concerning the mechanisms of the initial steps of particle formation in Hyytil, Finland. The initial analysis supports a hypotheses of formation of thermodynamically stable clusters (Kulmala et al., 2000), their subsequent growth by condensation of sulphuric acid molecules and further activation by soluble organic vapors, resulting in remarkably constant growth rates in the nucleation mode size regime. Much of the work has been done under EU-project CASOMIO. Horrak, U., Salm, J., and Tammet, H. (2000) J. Geophys. research D 105, 9291. Kulmala, M., Pirjola, L., and Mokel, J. M. (2000): Nature 404, 66. McMurry, P. H. (2000a): Atmos. Env. 34, 1959. Ramanathan, V., Crutzen, P. J., Kiehl, J. T., and Rosenfeld, D. (2001) Science 294, 2119. Stott, P. A et al.(2000) Science 290, 2133. Wichmann, H.-H., Peters, A. (2000): Phil. Trans. R. Soc. Lond. A. 358, 2751. Yu, F., and Turco, R. P. (2000): Geophys. Res. Lett. 27, 883.